I've been a lurker here for quite a while...I have a 1991 Nissan Maxima SE with 289,000 miles on it. My dad bought it brand new in 1991.

It runs and drives great, with one exception....

I had my coolant system flushed by my local mechanic last year who said he flushed a lot of sediment out of my heater core. Since it came back from the mechanic about a year ago the heater output hasn't been great. With a recent cold snap I notice the temp gauge goes from just below level to full cold while driving down a long downhill on a country road driving 45-55mph and the outside temp is 33°. The heat also seems better with the car sitting and cools down once underway.

SO....my plan has been to bleed my cooling system as described in the TSB HA90-12 and or to use a Prestone bleed kit on the upper hose to the heater core as I've read other forum members do. I have not bypassed the heater water valve....yet. (should I?)

My question is: Do I need to replace the thermostat too? And if yes, do I use blue RTV or can I use a gasket like this one:
http://www.autozone.com/autozone/par...59134_0_11117_

ADDED IN EDIT:
Once the engine is warmed up, the upper heater core hose that is hooked to the water valve is plenty hot. The coolant enters the heater core through the upper hose, right? So, if I understand things correctly, that means the water valve is opening. I am headed outside now to flush some water through the heater core and bleed the system.

Thanks!

 

Well, I flushed my heater core with water from both directions and nothing but water and coolant came out. The interesting thing was when I jacked the front end of the car up to bleed it, there was coolant flow from left to right visible in the top of the radiator immediately after starting the car cold...not sure this is normal. The air blowing out of the vents is warm but not hot and cools down as soon as I begin to drive. We had a high temp today in the low 40s.

I picked up a thermostat at NAPA and am going to try that next.

 

Your first mistake was letting a mechanic flush your system. Likely this caused whatever issue you are having. You are sure you have your system full of coolant? If the temp gauge goes down when you are going downhill it could either mean you have a faulty sender which is possible considering it is 17 years old or your water pump is bad.

 

Thanks for the reply. It only gets cold going down a long hill like on the freeway at 70mph...or a long grade after a mountain pass...but only when it's cold outside.

I know the mechanic and he said they flushed the heater core and radiator with water and changed the coolant and lower radiator hose...the upper rad hose failed and I replaced it a couple of months before I took it to my mechanic to change the lower hose, that C-shaped hose and flush the heater core. My secretary has a 89 Maxima and her husband flushed her heater core and she said the heat was better afterward...so I had my mechanic check mine. I should have had him change the t-stat while they were doing all that.

I **think** the system is full of coolant...I jacked the front of the car way off the ground and shimmed the throttle stop to a 1700RPM and added coolant until it stopped bubbling and the radiator was full. Also, when I opened up the hoses to the heater core coolant came out.

The car is 21 years old, so anything is possible....the water pump and timing belt haven't been done for 100K...so they're due. The water pump doesn't leak...I'm not sure what else goes wrong with them...I would think with an impeller failure, it wouldn't work at all...I've never had anything other than bearing or seal failure on a water pump on all the vehicles I've owned.

Does anyone have a coolant flow diagram for the 3rd Gen Maxima?

 

Sure sounds like the thermostat is stuck opened. Since you see water flowing on the radiator hole, the water pump should be okay. Take care of the thermostat and re-try the bleeding procedure to make sure it runs ok.

And someone was able to a very descriptive page in regards to the coolant flow if you want to check it out.

http://www.cardomain.com/ride/748507...maxima/page-8/

I'm suprised you've driven the timing belt for 100K miles. The older style timing belt are supposed to be changed every 60K.

 

Thanks for the link jjbons25!

Now this has got me thinking that maybe I should go ahead and replace the timing belt, water pump, and t-stat at the same time.